However, Hong Kong's future high-speed train station, the West Kowloon Terminus, lacks a direct high-speed rail link to Chek Lap Kok Airport, which diminishes the attractiveness of the airport as a transit hub for high-speed train passengers from the mainland, he added.

"High-speed rail is a good thing for big airports. For small airports, it poses competition," said Liu Wu Jun, chief technical officer of the Shanghai Airport Authority.

For example, the airport in Wuxi city, Jiangsu province, previously had one daily flight to Beijing, but that has ceased, Liu said. Now, passengers from Wuxi take high-speed trains to the Hongqiao transport hub in Shanghai, where they take a plane to Beijing, Liu said.

There are now 70 flights from Hongqiao to Beijing every day, Liu added. "So passengers from Wuxi go to Hongqiao."

The Hongqiao transport hub includes a high-speed train station, subway connections, and an airport where 90 per cent of the flights are domestic.

Hongqiao now offers 230 travel tickets that combine high-speed rail with flights. Among them is a ticket for a high-speed train journey from Nanjing to Hongqiao plus a flight out of Hongqiao, said Liu. "This is good for passengers. If we don't have these combined tickets, they will go to other airports."

Last year, 10 per cent of travellers passing through Hongqiao were high-speed train passengers, Liu estimated. Those who buy combined high-speed train and air tickets via Hongqiao were mainly business travellers.

"There will definitely be more business travellers buying the combined tickets. Time is of the essence to them."

At Shanghai's other airport at Pudong, 90 per cent of the flights are international, so Pudong International Airport will not suffer from high-speed rail services, said JP Morgan analyst Karen Li.

"When the Beijing-Shanghai high-speed railway became operational in 2011, lots of people were concerned that it would cause a lot of traffic diversion. There was some traffic diversion but not that much," said Li.

Since the Beijing-Shanghai High-Speed Railway started in June 2011, the Beijing-Shanghai flight route has lost 3 to 5 per cent market share, she estimated.

But by the end of this year, when 3,500 kilometres of new high-speed rail links will be completed, the bigger network will make train travel more attractive, Li said.

After that, Beijing-Shanghai flights may lose market share of 30 to 40 per cent, while other mainland short-haul flights risk losing market share of up to 30 per cent, he warned.

In January, Air China, China Southern Airlines, China Eastern Airlines, and other mainland carriers, collectively reported one billion yuan (HK$1.23 billion) in losses, partly due to competition from high-speed rail.

To keep passengers from switching to high-speed trains, airlines have been offering steep discounts. For example, a one-way economy high-speed train ticket from Guangzhou to Wuhan typically costs 463.50 yuan, but mainland airlines are offering discounted air tickets for this route priced as low as 235 yuan.

Ironically, the diversion of traffic by the rail services will benefit Beijing airport, due to the airport's congestion in domestic flights, said Li.

"It will be positive for Beijing airport, because the profitability of domestic flight charges is lower than for international flights. It will be bad for domestic airlines because most of their business is domestic," he said.

High-speed rail will have a huge effect on flights of less than 800km, but little impact on long-haul flights above 800km, said Zheng. After the network is completed in 2020, 60 per cent of journeys of less than 800km will be on high-speed trains, Zheng said.